Waterski with improved torsional characteristics

ABSTRACT

A water ski with improved torsional characteristics delimited on the upper side by the upper surface, on the bottom side by a concave surface and on the sides by curvilinear sidewalls. The ski has a tip at the front side and a tail on the rear side. On the central part of the ski, on the upper surface, a longitudinal groove is provided having a flat bottom surface connected with vertical longitudinal sides of the groove by rounded surfaces, wherein the groove decreases torsional and flexural strength, which depth and width reduce torsional and flexural stiffness of the central part of the ski. The width of the groove may be 30-80% of the width of the ski, and the depth of the groove may be 20-95% of the thickness of the ski.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is the US National Stage of International Application No. PCT/RS2015/000008 filed Mar. 19, 2015, and claims the benefit thereof. The International Application claims the benefit of Serbian Application II-2014/0130 filed Mar. 21, 2014. All of the applications are incorporated by reference in their entirety.

TECHNICAL FIELD

The present invention pertains to the group of equipment for water sports and recreation, primarily skis for slalom but also other similar equipment for water skiing. Implementation of the present invention is not limited to one kind of skiing and one type of skis, therefore, the water skis according to the present invention can be used for various kinds of water skiing, for example for slalom, jumps, tricks and other. The present invention is intended for a wide circle of water skiers as it can be used for both recreational and competitive skiing. One of the advantages of the invention that will be described in detail herein below is that it extends the circle of water ski users who will be able to improve their results, take part in competitions and ski in the manner which, so far, has been possible only through disproportional amount of training and investment.

In accordance with the International Patent Classification (2013 issue) the invention is classified into the group B 63 B 35/00 that includes vessels or floating structures adapted for special purposes, and pertains to the subgroup B 63 B 35/81 that includes water skis and water ski sleds.

BACKGROUND

It is known that any movement of a solid object on the water or through the water has respective hydro-dynamic characteristics of such movement and that due to the skier's movement and drag there is complex load acting on the object that moves on the water or through the water. When water skiing, there is a complex load acting on the ski that is the sum of various loads among which torsion, flexure, pressure and other. Predominant load components are torsion and flexure, in particular at some moments of skiing so that in a simplified ski load model under certain conditions of ski riding it may be said that the ski is primarily under the load of torsion and flexure or that only these two loads are taken into account. The load to which a ski is exposed during a ride is transmitted via foot bindings, skier's body and tow rope to the towboat or towing system and such load action causes a whole series of adverse consequences. The adverse consequences reflect themselves in that the ski control is more difficult, more strength and physical fitness is required from a skier, and impact the boat or towing system speed, activation of automatic speed control (Perfect Pass or Zero Off) of the boat or towing system, etcetera. The aim of the present invention is a ski design that would eliminate a large portion of these adverse ski load consequences.

On the basis of the above stated, the problem solved by a ski according to the present invention can be defined as follows: How to design a water ski of reduced stiffness or, in other words, of increased elasticity which by elastic (reversible) deformation, in this case primarily by torsion, adapts itself to hydro-dynamic skiing conditions, primarily to flexural load, without jeopardising in the process the necessary ski strength, that is, without increasing the probability of damage or break and thereby the safety of skier, either. Adapting to hydro-dynamic skiing conditions accomplishes a series of advantages that will be described in more detain herein below. More precisely, the technical problem can be also defined as follows: How to design a water ski of reduced stiffness in which it is provided for different parts of the ski viewed lengthwise, i.e. the tip, central part and tail of the ski, to differently deform elastically by twisting at different angles around the longitudinal axis in order to adapt to the hydro-dynamic conditions of movement on water while at the same time making ski control more easy without risky decrease of the ski strength.

There are several renowned water ski manufacturers (Goode, Connely, D3, Radar, etc.) present in the international market. Geometry of the upper surface of the water skis offered by these manufacturers is such that there are no deep grooves that would reach the bottom ski surface (laminate) which would have significant effect to the mechanical characteristics of a ski. The purpose of the grooves existing on the skis present in the market, that are shallow, respectively have a depth of up to 2-3 mm, is more of a decorative nature and they do not have any known effect on the mechanical characteristics of the skis. It is not known that any experiments or tests were performed on skis with a deep and relatively wide groove which significantly influences the mechanical characteristics of skis. In contrast to that, the problem defined above and known to ski manufacturers was being solved through the selection of material, fitting of parts into the ski structure, as well as a ski structure already factory flexed on the left or right side.

Searching patent documentation, the following documents have been found.

Patent document U.S. Pat. No. 3,731,328 discloses a water ski having depressed top surface. The depression should provide a ski having new means for securing foot retainers. The top surface or the top portion of the ski is recessed at location immediately in front of the first foot retainer and extends back toward the rear of the ski ending immediately behind the second foot retainer. For fitting the edges of foot retainer two channels are formed on the vertical longitudinal sides of the depression and channels connect vertical sides of the depression with the bottom surface of the depression. The edges of foot retainers are held in the channels by plates. The dimensions of depression are chosen according the aim of the invention i.e. to provide new mean for securing foot retainers on the ski.

U.S. Pat. No. 3,861,698 discloses a combination of snowshoe and ski. The basic structure of described invention is hollow plastic envelope with longitudinal corrugation along its upper portion. The depressed part on the upper portion of the ski show is obtained as a curved shape of upper part of hollow envelope and not as a recess in the cross section of full material. The longitudinal corrugation along its upper portion, as well as transversal corrugation along its bottom portion are provided to rigidify the snowshoe i.e. the hollow envelope structure.

Patent document U.S. Pat. No. 6,533,625 B1 (P. E. Taylor) describes a water ski where in its bottom surface there are several grooves formed from the central concave surface (“concave tunnel”) towards the ski outward. The grooves are in a fishbone-like arrangement. Purpose of the grooves is to provide more acceleration for the water skier and improve stability of the ski. Differences between the grooves in the prior art ski and the groove of the ski according to the present invention are obvious as the grooves in the prior art ski are made in the bottom surface of the ski and have different position from the groove according to the present invention that is made on the upper surface of the ski and the prior art ski solves quite other technical problem as opposed to the ski according to the present invention.

Patent document U.S. Pat. No. 5,788,548 (T. D. Hjaltason) describes a mono-ski having a narrow slot extending rearward from the ski toe to the central part of the ski. The narrow slot in the front end of the ski is intended for locating a tow rope that is attached before the skiing starts, respectively immediately before the towing starts. With respect to its structure and objective the prior art ski is completely different from the ski according to the present invention because it involves a slot and not a groove and because the aim of the prior invention is quite different, i.e. taking of a suitable tow rope position at the start position of a skier.

Patent document U.S. Pat. No. 5,525,083 (R. Reich) describes a ski narrowed in the central part to which, beside other characteristics, plates are secured for “adjusting stiffness and flexibility”. The plates are of elongated shape and have a pair of longitudinal slots in the central part that serve for fixing one or more plates to the ski with screws. Here, the function of slots is not to affect the stiffness, that is, they do not serve to reduce it but to provide for plate fixing. Adding the plates increases stiffness thus a significant increase of the stiffness may be expected because it is stated in the document that the plates are made of metal, preferably aluminium alloys. As generally known, flexural stiffness is the product of a shearing modulus and the torsional moment of inertia. The shearing modulus is a material characteristic and metals have relatively high shearing modulus so, therefore, significant increase of the ski stiffness may be expected due to the attachment of metal plates. Hence, it is apparent that the aim to be achieved by the prior art invention is contrary to the aim of the ski from the present patent application and that the means for achieving the aims are different. In addition, the structure is complicated as it comprises a larger number of parts.

Patent document U.S. Pat. No. 6,056,311 (K. F. Leung) describes a ski which, as the most important characteristic, has one or a plurality of longitudinal slots where transverse wings are mounted. Such structure should achieve the increase of uplift force acting vertically on the ski thanks to the water flow trough a slot between specifically designed transversal wings. Besides such ski being very complicated as it has a large number of structural parts and, for that reason, also expensive, possible change of torsional properties that could be achieved by formation of one or a plurality of longitudinal slots is annulled by the presence of a lattice structure formed by transversal wings. It can be seen from the above explained that the problem which is solved and the structure of the invention according to the prior art are completely different from the subject invention.

SUMMARY

A water ski with improved torsional characteristics according to embodiments has a central part, a tail and a tip, as standard in the skis known in the state of art. The ski is on the upper side delimited with the upper surface and on the bottom side with the bottom surface. On the lateral sides the ski is delimited with a side wall or a sidewall surface. The side wall or sidewall surface may be of various shapes or composed of several different surfaces. Sidewall surfaces follow a curve and narrow the ski at the tip and tail. The sidewall surface in the disclosed example of the embodiment is substantially an oblique flat surface but may also be a surface the cross-section of which is a curve. The ski may also have a slot for a fin (not shown here) located at the ski tail. Different foot binders and/or supports may be used as a connection between a skier and the ski. All of the elements identified so far may have different shapes and combinations thereof that are known to those skilled in the art and that are present in the skies on the market. The water ski according to embodiments, unlike known skis, has a longitudinal groove formed on the upper surface. The groove is formed along the length of the central part of the ski and its length may be in the range of 20% to 50%, preferably 30% to 40% of the ski length thus defining the central part of a ski with reduced torsional stiffness. The groove width is from 30% to 80%, preferably 40% to 50% of the ski width and the groove depth may go to the last layer of the material (laminate) in case when the ski is of a laminated structure or a combined structure. Since a ski may be executed as a monolith or from a plurality of material layer or a core and an outer layer, it is important to note that the depth of a groove is 20% to 95%, preferably 80% to 95% of the ski thickness. The said concepts of the width, depth and length of the groove refer to mean values depending on the actual execution of the ski groove, i.e. the shape of walls, bottom and end of the groove. More precisely, a mean value is the arithmetic mean of the minimum and maximum measured values of length, width and depth. The bottom surface of the groove is flat (i.e., without corrugations or ribs) whereas the groove walls may be flat, perpendicular to the upper surface of the ski, at inclination, rounded or of some other suitable shape. Ends of the groove, the front and rear end, may be in the form of a semi-circle, semi-ellipse, letter “V” or any other suitable form. Mathematical models for the calculation of the stiffness reduction achieved by this groove are relatively complex considering that they deal with a space- and time-related fluctuation of continuous load and not a constant concentrated force, relatively complex cross-section form, as well as other factors due to which simplified calculation models would not render results that are achieved by testing. Whereas the length of groove determines the part of a ski in which torsional characteristics are changed, the width and depth of the groove are of crucial importance as to the extent of influence on torsional characteristics, that is considerable decreasing the ski stiffness to twisting in that part of the ski and not posing threat to the ski strength, that is not increasing probability of a breaking or damage. It is known that the angle of twist is directly proportionate to the moment of torsion and the distance of cross-section from the fixing point, and inversely proportionate to stiffness. Stiffness is the product of a shearing modulus and torsional moment of inertia. Whereas the shearing modulus is a characteristic of the type of material, the torsional moment of inertia depends on the geometry (shape) and size of cross-section. Providing a groove the cross-section subject to torsional load is reduced and, in that way, also the torsional moment of inertia and stiffness. Owing to that the deformation is increased, that is the angle of twist in the part of ski where the groove is provided. Shallow grooves provided only for decoration of the ski upper surface and of no effect on the mechanical characteristics of a ski, in particular the torsional stiffness, cannot be brought in correlation with the ski according to the present invention. Grooves which dimensions would exceed the above specified, on the other hand, would result in a risky decrease of strength, i.e. the increase of probability of ski damage or breaking. It follows from the above that the dimension relations represent the substantial characteristics of the present invention and as such they are also the subject of protection. The described groove may be provided on various types of skis and boards.

Existence of a sufficiently deep and wide groove has the effect on the torsional stiffness of the central part of a ski, respectively that part of a ski on which the groove extends, and results in a bigger deformation, i.e. bigger angle of twist in that part of the ski whereas the parts where a groove is not provided, front end with a tip and rear end with a tail, retain their original stiffness and at the same load deform, that is twist at an angle different to the one of the central part of the ski. It is understood that here are discussed solely elastic deformations. It is also clear that a ski is exposed to a complex load where torsion and flexure components are the most expressed and thus only torsional and flexural loads are discussed in the simplified load model. It is evident from this explanation that by providing a groove a ski is obtained in which different parts viewed lengthwise have different torsional stiffness and that the ski does not deform equally in all its parts but that the torsion in the central ski part is facilitated by the provided groove. Such a ski makes ski control easier because the ski tipping on the sidewall is easier and it is easier to take and keep the position, i.e. direction of skiing with respect to a towing device. This solves yet another issue present in water skiing. Making turns, e.g. in slalom, results in a greater load and force in the rope that acts in the skier's direction, in other words pulls back (decelerates) the towboat. In towboats that are equipped with automatic speed control the boat speed increases and this again results in a greater strain and more difficult water skiing control. Besides the indicated case it is clear that the advantages of a ski with improved torsional characteristics can be used by all categories of water skiers, from the beginners through recreational skiers of different physical strength to competitors. Advantages offered by a ski according to the present invention are manifested in various kinds of water skiing: slalom, jumps and tricks, wake-boarding and other.

BRIEF DESCRIPTION OF DRAWINGS

Water skies with improved torsional characteristics will be described herein below based on an example of embodiment and the attached drawings. The embodiment example disclosed in the detailed description and illustrated in the drawings is a non-limiting example of the invention the protection thereof is requested in the claims. The example is given for a better understanding of the invention. The following is shown in the drawings:

FIG. 1 is an axonometric projection of a ski embodiment according to the invention;

FIG. 2 is a longitudinal sectional view of the ski embodiment taken along line A-A in FIG. 3;

FIG. 3 is a top view of the ski embodiment;

FIG. 4 is a cross-section view of the ski embodiment taken along line B-B in FIG. 4. In this embodiment the ski embodies a core and an outer layer, however, the invention is not limited to only this embodiment.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows an embodiment of the ski 1 with improved torsional characteristics according to the invention. The invention refers not only to the presented embodiment of the water ski 1 but also to the other types of skis. The ski 1 is on the upper side delimited by the upper surface 2 which in its major part is in fact a curved surface and bent upwards in the region of the tip 4 of the ski 1. From the bottom side the ski 1 is delimited by the bottom surface 3 (also see FIG. 4). The bottom surface 3 is concave, respectively curved outward to the body of the ski 1 viewed from the cross-section and, viewed lengthwise, the bottom surface 3 of the ski 1 is curved being convex to the outside (from the skier's perspective). On the side, the ski 1 is delimited by the sidewalls 7 which follow the form of a double curvature and can be of a different shape depending on the type of the ski 1. The sidewalls 7 narrow the ski 7 at the front end where there is the tip 4 and at the rear end where there is the tail 5. The sidewall at the tail can follow a straight line perpendicular to the longitudinal axis of the ski 1. In the disclosed embodiment the ski 1 at the tail 5 has a slot 9 for the attachment of a fin. In this embodiment, on the upper surface 2 openings are also provided for the attachment of additional equipment. All the elements described so far are characteristic of the skis available in the market and known to those skilled in this field of art. The ski 1 according to the invention has a longitudinal grove 6 provided on the upper side which longitudinal axis coincides with the longitudinal axis of the ski 1 (FIGS. 1 and 3). The longitudinal groove 6 runs on the central part of the length of the ski 1 and it is not provided on the rear end of the ski 1 with the tail 5 or the front end of ski 1 with the tip 4. The length of the longitudinal groove 6 is 20 to 50% total length of the ski 1 and, viewed lengthwise, the part of the ski 1 on which the groove 6 is provided represents the part of the ski 1 that more easily elastically deforms to torsion and flexure or, in other words, has different torsional stiffness from the part of the ski 1 where the groove 6 is not provided. More suitable range of the length of the ski 1 where the groove 6 is provided is 30 to 40% of the ski length. Size of the length of the ski 1 provided with the groove 6 depends on the area of the ski 1 that requires altered torsional characteristics of the ski 1, that is, in which part of the ski 1 is required a reduced torsional stiffness, which is conditioned by the intended use of the ski. In any case, within the scope of the present invention, various variations of the length of the groove 6 are possible in accordance with the intended use and type of the ski 1. The form of the cross-section of the groove 6 of the ski 1 (FIG. 4) is preferably trapezoid as shown in FIG. 4 but it may be rectangular or have any other suitable form. The form of the cross-section of the groove 6 depends on the shape of the walls of the groove 6; they may be at a right angle with respect to the upper surface 2 or slanting thereto. The width and depth of the groove 6 of the ski 1 (see FIGS. 2 and 4, longitudinal section and cross-section) are of particular importance, i.e. the ratio between the depth of the groove 6 and the thickness of the ski 1, as well as the ratio between the width of the groove 6 and the width of the ski 1. The depth of the groove 6 of the ski 1 is in the range from 20 to 95%, preferably 80 to 95% of the thickness of the ski 1 at the place of the groove 6, i.e. down to the last layer of the material (laminate). The width of the groove 6 is in the range from 30 to 80%, preferably 40 to 50% of the width of the ski 1 at the place of the groove 6. The groove length, width and depth are mean values depending on the shape of the groove wall and groove end, as already stated above. The sizes of the width and depth of the groove 6 specified herein are of crucial importance for the application of the ski 1 according to an embodiment and represent the results of the tests of the ski 1 prototype. Actual depth and width of the groove 6 from within the identified range depend on the type and intended use of the ski 1 but also on the material from which the ski 1 is made. Is the depth and width of the groove 6 would be smaller that the specified dimension ratio, the stiffness of the ski 1 to torsion would not be considerably reduced and the ski 1 would not function in the desired manner. In such case it could be said that shallow and narrow grooves represent decorative grooves and do not have effect on the ride characteristics of the ski 1, that is its functioning, and that the skier would not feel a difference between a ski of a full profile and the ski 1 according to the invention. The front and rear end of the groove 6 may be of a different or same shape. In addition to the already presented semicircular shape of the end of the groove 6, a semi-elliptic or angular shape of the end of the groove 6 is also suitable; however, other suitable forms may also be used.

The use and functioning of the water skis 1 according to the invention are described herein below. In water skiing the ski 1 is exposed to a complex load which is the consequence of the traction force and hydrodynamic conditions prevailing during the movement of an object on or through water and there is the transfer of force in both directions from the water, via the skis 1, the skier, and rote to the towboat. From the analysis of the complex load and from the experience it is known that one of the dominant components of the load is torsion and flexure. This is particularly noticeable in specific moments of the ride, e.g. when making turns or in slalom. To control the skis 1 it is necessary that the skier gets inclined with respect to the water surface. It is also favourable that the ski 1 adapts to the hydrodynamic conditions of skiing. In order for this to be achieved it is necessary that the rear end of the ski 1 with the tail 5, the central part of the ski 1 and the rear end with tip 4 of the ski 1 have different torsional characteristics, more precisely that the central part of the ski has reduced stiffness to elastic torsional deformation. Application of various calculation models is possible although significant simplification of the model would have to be made relative to the actual skiing conditions and the load action to the ski 1; however, the following is indisputable. It is known from the material resistance that deformation to twisting (torsion), i.e. the angle of twist is directly proportional to the torsional moment, the force acting length, and inversely proportional to the cross-section characteristic, i.e. the torsional moment of inertia that depends on the shape and dimensions and, hence, also the area cross-section. The angle of twist depends also on the constant that represents the characteristic of the material, i.e. it is inversely proportionate to the shearing modulus. The inventors have recognized decreasing the cross-section area where the angle of twist is measured results in a decrease of the respective cross-section characteristic and an increase of the angle of twist. In this way, providing of the groove 6 of sufficient size, i.e. sufficient depth and width in the central part of the ski 1 results in the change of its torsional properties, i.e. to their improvement in terms of the control and easier skiing on water. The front end of the ski 1 where there is the tip 4 and the rear end of the ski 1 where there is the tail 5 and where the longitudinal grove 6 is not provided retain their original torsional properties so that at the same load there is a different torsion of the different parts of the ski 1. In other words, the groove or depression on the top surface of the ski changes cross-sectional characteristics of the ski making it more flexible and reducing its stiffness on torsion and flexion and chosen dimensions are dependent on mechanical characteristics. Also, as can be seen in the Figures and as described herein, the vertical longitudinal sides of the groove are connected with the bottom surface of the groove by rounded surface and it has no channels which can be a cause of the stress concentration and lead to damage of the ski. The direct consequence of different, to a certain degree less dependent, angles of twist of the different parts of the ski 1 results in easier controllability of the skis 1 and better adaptation of the ski 1 to the current hydrodynamic conditions of the movement on water. The ride of the ski 1 is also easier because it is easier to take a desired angle between the ski 1 and the water surface. The control of the ski 1 is made easier; it requires less physical effort because tipping of the ski 1 to lateral surface is facilitated; it is easier to take position, i.e. the skiing direction and angle with respect to the towing device. In this way the skiing is made easier in particular to less strong skiers and becomes more accessible to women and younger persons. The ski 1 with improved torsional characteristics can be used by all categories of water skiers from the beginners through recreational skiers of various physical strengths to competitors. Advantages offered by the ski 1 according to the invention are manifested in various kinds of skiing: slalom, jumps, tricks, and other. It is necessary to emphasise once again that the relationships between the dimensions of the groove 6 and the overall dimensions of the ski 1 represent the intrinsic characteristic of the present invention, that they are defined on the basis of tests and that they inevitably represent the subject of protection. Shallow grooves, i.e. grooves of smaller dimensions provided only for the purpose of decoration of the upper ski surface and with no effect on the mechanical characteristics of a ski, in particular torsional strength, do not provide for the functioning of the ski 1 according to the present invention in the desired manner. On the other hand, grooves of dimensions greater that those specified would result in a risky decrease of strength, i.e. increase of the probability of ski damage or break. The ski 1 according to the invention is made from materials that are customarily applied in the manufacture of water skis such as fibreglass, graphite and others, not excluding the use of composite laminates as innovative materials. The water ski 1 according to the invention is made by standard technologies known to those skilled in this art so that big investments into new equipment are not necessary for the changeover to production of the ski 1 according to the present invention. 

1-5. (canceled)
 6. A water ski with improved torsional characteristics, the water ski having a given thickness and a given width, comprising: an upper side having a curved upper surface; a bottom side having a concave surface; sides having curvilinear sidewalls configured to narrow a front end and a rear end of the water ski; wherein said water ski has a tip at the front end and a tail at the rear end, character-ized in that a central part of the water ski on the upper surface is provided with a longitudinal groove, the groove having a flat bottom surface connected with vertical longitudinal sides of the groove by rounded surfaces, wherein the groove decreases torsional and flexural strength of the central part of the water ski; and wherein the groove has a width that is 30-80% of the width of the water ski; and a depth that is 20-95% of the thickness of the water ski.
 7. The water ski according to claim 1, having a given total length, wherein the groove is 20-50% of the total length of the water ski.
 8. The water ski according to claim 7, wherein the groove is 30-40% of the total length of the water ski.
 9. The water ski with improved torsional characteristics according to claim 1 or 2, wherein the groove has a trapezoid cross-section.
 10. The water ski with improved torsional characteristics according to claim 1 or 2, wherein the groove has a rectangular cross-section.
 11. The water ski with improved torsional characteristics according to any of the above claims wherein the ends of the groove are of the same or of a different shape and in that the ends of the groove are in the form of a semi-circle or angle.
 12. The water ski of claim 1, wherein the width of the groove is 40-50% of the width of the central part of the water ski.
 13. The water ski of claim 1, wherein the groove has a depth corresponding to 80-95% of the thickness of the water ski. 